(1) Field of the Invention
The present invention relates to an optical writing device and an image forming device that executes writing onto a photoreceptor by light.
(2) Description of the Related Art
Japanese Patent Application Publication No. 2004-98317 is one example of conventional technology disclosing an optical head utilized in image forming devices such as a printer. The optical head includes a rod lens array that collects light emitted by a plurality of light-emitting elements, and exposes a photoreceptor to the light.
FIG. 16 is a schematic planar view illustrating a positional relation of the light-emitting elements and the rod lens array in the optical head pertaining to the exemplar conventional technology discussed above, when the light-emitting elements are viewed from the photoreceptor through the rod lens array.
As illustrated in FIG. 16, in the optical head pertaining to the exemplar conventional technology, a plurality of light-emitting elements 900 are arranged two-dimensionally. That is, the light-emitting elements 900 are arranged to form lines along a main scanning direction. Each of the lines is a light-emitting element array 901, and a plurality of light-emitting element arrays 901 are arranged along a sub scanning direction. A rod lens array 910 collects light emitted from the light-emitting elements 900.
The rod lens array 910 has an elongated shape. The rod lens array 910 is includes a large number of rod lenses 911 each having a diameter larger than the diameter of one light-emitting element 900. The rod lenses 911 are arranged in a zigzag pattern along the main scanning direction.
Due to this structure, the rod lens array 910 has an optical characteristic such that different portions of the rod lens array 910 have different light transmittance levels. Because of this, even when light beams emitted from the two-dimensionally arranged light-emitting elements 900 emit light beams with the same light amount, after the light beams pass through the rod lens array 910, the light beams arrive at regions of a surface of a photoreceptor with different light amounts (i.e., the regions have different light exposure amounts). This results in forming of an image having uneven density.
In order to suppress such uneven density, the above-cited Japanese Patent Application Publication No. 2004-98317 discloses a method for correcting driving currents supplied to light-emitting elements.
The configuration of Japanese Patent Application Publication No. 2004-98317 requires a correction circuit for correcting driving currents supplied to light-emitting elements to have a large dynamic range. The reason for this is described in detail in the following utilizing FIG. 17.
FIG. 17 includes graphs illustrating one example of difference in light amounts of light beams having passed through a rod lens array, in a hypothetical case where light beams emitted from all light-emitting elements of two light-emitting element arrays have the same light amount before passing through the rod lens array. The horizontal axis shows positions along the main scanning direction, and the vertical axis shows light amounts of light beams having passed through the rod lens array.
Graph 931 shows light amounts of light beams emitted from the light-emitting elements 900 of a light-emitting element array 901a. The light-emitting element array 901a is one of the five light-emitting element arrays 901 illustrated in FIG. 16, and is disposed at the center in the sub scanning direction. Graph 931 shows that light beams from the light-emitting elements 900 of the light-emitting element array 901a have light amounts varying from one other after passing through the rod lens array, and that the difference in light amounts indicates a periodical pattern along the main scanning direction. Such light amount difference is hereinafter referred to as a “light amount difference along the main scanning direction”. Graph 932 shows that the light amount difference along the main scanning direction also occurs with respect to the light beams emitted from the light-emitting elements 900 of a light-emitting element array 901b. The light-emitting element array 901b is one of the two light-emitting element arrays disposed farthest from the center in the sub scanning direction.
In graphs 931 and 932, “ΔA” indicates levels of the light amount difference along the main scanning direction occurring with respect to the light-emitting element arrays 901a and 901b, respectively.
Furthermore, graphs 931 and 932 indicate that after having passed through the rod lens array, the light beams from the light-emitting element array 901a have greater light amounts than the light beams from the light-emitting element array 901b. This difference in light amounts is illustrated as shift amount AB in the vertical direction in FIG. 17. This difference in light amounts is due to the light-emitting element arrays 901a, 901b having different positions along the sub scanning direction relative to the rod lens array 910, and is hereinafter referred to as a “light amount difference along the sub scanning direction”. The light amount difference along the main scanning direction and the light amount difference along the sub scanning direction are caused by the structure of the rod lens array 910 described above and the consequent optical characteristic of the rod lens array 910.
Meanwhile, Japanese Patent Application Publication No. 2004-98317 discloses correcting driving currents supplied to light-emitting elements by correcting values of light amount signals supplied to driving circuits supplying driving currents to light-emitting elements. A light amount signal supplied to a driving circuit indicates the amount of light to be emitted by a corresponding light-emitting element.
When employing this method to suppress both the light amount difference along the main scanning direction and the light amount difference along the sub scanning direction, the light amount signals need to be capable of indicating the maximum light amount in graph 931 and the minimum light amount in graph 932. That is, the light amount signals need to be variable within a relatively great range, corresponding to difference ΔC between the maximum light amount in graph 931 and the minimum light amount in graph 932. Accordingly, a relatively great dynamic range is required for correcting the driving currents, unfortunately.
A greater dynamic range results in, for instance, a necessity of providing a greater number of bits to a digital signal representing light emission amounts of light-emitting elements. Further, the greater the number of bits of the digital signal, the larger a correction circuit becomes, because the number of elements in the correction circuit, such as a logic gate for executing processing such as D/A conversion to the digital signals, increases accordingly. This results in expensive semiconductor elements, such as an IC including the correction circuit, being required.
The above problem is not specific to cases where lenses are used, one example of which being a case where a rod lens array is used. That is, the same problem may occur with any optical writing device in which the light amount difference along the main scanning direction and the light amount difference along the sub scanning direction occur at a photoreceptor.